Monday, January 26, 2009

Most Neal Stephenson fans probably read his book Anathem last fall, but I just finished. There are many reviews available, so I won’t add one here, but I got a big charge out of some of the philosophical ideas Stephenson put in the book.

In the course of the novel, Stephenson proposes an interesting relationship between platonic truths, the multiverse, and human rationality/consciousness, which I have enjoyed comparing to my own ideas (discussion below the fold).The main idea is that the platonic realm is not a single transcendent world out there, but rather the source of platonic ideas is to be found in the multiverse (called the “polycosm” in the book; the overall theory is called “complex protism” where protism=Platonism and “simple protism” would be the positing of a single platonic realm).

The core of this idea is one I endorse here in the non-fictional realm, and it was very cool to see it in the novel. Modal judgments about what is possible and what is necessary play a crucial role in our reasoning. At the same time, both philosophers and theoretical physicists have proposed that our world is one member of a set of many possible worlds. If we propose that the logical possibilities we explore with our minds are identical with real possibilities present in the multiverse, then the multiverse can be seen as the source of our rationality, and of our knowledge of abstract truths. (The philosophical term for the match-up between logically possible worlds and metaphysically possible worlds is modal rationalism).

But how does a human mind access the multiverse? First, let me note that the type of multiverse being proposed in the book is inspired by the many-worlds interpretation (MWI) of quantum mechanics (QM). So the solution is to have the mind exploit the quantum realm. There’s a dialogue between the characters Orolo and Erasmus (pp. 543-548) where they discuss how the presence of the same person’s brain in multiple adjoining and interfering worlds gives the brain access to possibilities. Stephenson’s characters later make the point (which I liked) that it isn’t that the human brain is the only thing which is in contact with possibilities; one should assume everything in the world experiences some contact, but it is in our own brains that we can best see the evidence manifested (pp. 690-92).

I’ve come at this in similar spirit, with one apparent difference when it comes to interpreting QM. I believe that our world is comprised of quantum measurement “collapse” events, which each embody the actualization of one of many possibilities. The fact that we humans are ultimately grounded in quantum-level events gives us a kind of direct acquaintance with possibilia in everything we do. That makes our ability to have modal knowledge intelligible, even though we don’t yet know the manner in which our brain/body system leverages this presumably micro-level contact into macro-level knowledge.

It appears Stephenson’s idea is to try to preserve the MWI (which attempts to do away with measurement events), while allowing contact between parallel worlds to be exploited by the mind. MWI would not typically be seen as allowing any contact. Also, usually in MWI, worlds branch (what we think of as a measurement is a splitting of worlds), whereas Stephenson wants to keep worlds in parallel. Of course, positing contacts between parallel worlds is very helpful for the creating the exciting parts of the novel which involve characters and spaceships moving between worlds.

The other aspect of Stephenson’s multiverse which is interesting is that the informational contact between worlds has a flow, where some worlds are upstream or downstream from one’s own world. It seems he places a single purely platonic world at the source of the information flow (whereas I would follow the usual philosophical tradition of identifying logical and mathmatically necessary truths to be those things true in all worlds). I’m not sure this makes sense or was philosophically motivated, but it was a neat twist.

There are allusions to many other philosophical and scientific ideas in the book, and Stephenson discusses many of the sources which inspired these in this acknowledgments page on his website. There’s more to follow up on there – one philosopher he discusses who I have not read is Edward N. Zalta.

Thursday, January 22, 2009

A very good talk by Lee Smolin at a Perimeter Institute conference last fall. I thought he did a very good job in explaining how physics got into the practice of viewing time in a geometric fashion (with no important role for the present moment) and why this will not work when formulating a theory of the universe as a whole.

Tuesday, January 06, 2009

Quantum Aspects of Life is a collection of papers edited by Derek Abbott, Paul Davies, and Arun K. Pati which was recently published by the Imperial College Press (this review refers to the paperback edition). The target topic of the book is the role played by quantum mechanics (QM) in living things. Actually, we need to be more specific than that, since it’s accepted that all biology is based on chemistry, and chemistry is inherently based on quantum physics. In the book, the key distinction is described in a couple of different ways: in the foreword, Sir Roger Penrose distinguishes between “strongly” and “weakly” quantum mechanical features utilized by living things; the editors’ preface says the intent is to address the question of “whether quantum mechanics plays a non-trivial role in biology” (p.xiii). In each case the phenomena in question are those which are distinctively quantum mechanical: superposition, entanglement, tunneling, etc.

The volume is very welcome, since the topic seems to demand more focus than it has received. Confirming a significant quantum role could have a huge impact – both on the practical pursuit of biology and the philosophical perspective we take on the nature of life and mind.There has long been a good circumstantial case to be made that the remarkable nature of non-trivial QM effects may serve to help explain the remarkable capabilities of biological systems. The foreword, preface, and Davies’ opening chapter all invoke Schrödinger’s 1944 book What is Life? as the ur-text exploring this idea. However, experimental confirmation of QM’s role in life didn’t follow, and molecular biology experienced huge growth and success nonetheless. Again repeating themselves a bit, the editors, Penrose and Davies each lament that the science of the intervening decades has been dominated by the “ball-and-stick” model of chemistry. They seem to be implying that scientists (for institutional reasons perhaps) haven’t been bothering to look for quantum effects. Of course, a hurdle for the idea, not as explicitly evident in Schrödinger’s time, is the phenomena of environmental decoherence. An issue running through the book is the challenge posed by the fact that maintaining quantum coherence is very difficult even in carefully controlled experimental settings.

While experimental confirmation of non-trivial QM effects in biology has indeed been elusive, it has not been absent, and a recent result strikes me as not only important, but possibly seminal. I’m referring to the 2007 Engel, et.al. paper in Nature which showed the utilization of quantum coherence in photosynthesis (please see the post “Quantum Biology Goes Mainstream” for links). The timing of this result’s publication was such that it either barely pre-dated or else post-dated the submission of papers to Quantum Aspects of Life. As I read the book, I was often thinking about how this result might change the debate as we move forward: it not only showed the utilization of QM in one of the core processes in biology, it also showed the engineering challenge (and attendant resource demands) which are involved in confirming the presence of such a phenomenon. There are quite a number of papers out there which present theoretical models which postulate QM effects to answer outstanding questions in biology -- here’s a good example of this sort of paper I saw recently – but confirming a theoretical result is another story.

Quantum Aspects of Life begins with a nice foreword by Penrose, whose perspective will be familiar to those who have read his books. He thinks the human mind will demand a quantum-derived explanatory account (which he thinks will be linked to a future theory of quantum gravity). Now, environmental decoherence becomes an increasingly strong obstacle as distances lengthen, so quantum computing in the human brain seems unlikely to be a result of quantum coherence extending across neuronal assemblies. Penrose, and his collaborator Stuart Hameroff (who also contributes a paper to the volume), think, however, that quantum effects may be effected through intra-cellular structures (microtubules) thereby creating an avenue for large-scale computation. Penrose has the following thoughtful observation: “Indeed, there is no question that if the brain does make use of such “strongly” quantum-mechanical phenomena, it must do so through the agency of some very sophisticated organization. (p.viii)” Life and mind might combine small-scale quantum effects with (classically describable) organizational structure.

Chapter one, by Paul Davies, and Chapter three, by Jim Al-Khalili and Johnjoe McFadden both focus on the possible role QM may have played in the origin of life on earth (OOL). Life may have learned to exploit QM phenomena via natural selection after a classically explainable beginning, but it seems plausible that QM may have been key to life from the start. The complexity of the building blocks of life (as we know it) make it statistically very unlikely that the components randomly came together in a primordial soup as was once suspected. Davies explores a model of a quantum replicator as a precursor; both he and Al-Khalili/McFadden discuss the possibility of a quantum-coherent search algorithm which helped lead to an early replicator. These are interesting ideas, although probably a long way from being confirmed or falsified (I have prior posts on work by Davies and McFadden here and here, respectively).

Chapter 2 is from Seth Lloyd, who stays at 40,000 feet by discussing generally how complexity should be expected to arise in a universe which is inherently quantum-mechanical (see my several posts on Lloyd’s “universe as quantum computer” thesis here).

Models of how photosynthesis likely exploits coherence is the subject of Chapter 4 (with the Engel paper validating broadly the idea); the authors of Chapter 5 present models to help quantify the impact of environmental decoherence in biological contexts. This work seems to sharply delimit the potential for coherence; however other authors argue that dynamic systems can foster insulated sub-spaces larger than what would otherwise seem likely.

A quantum role in DNA mutation and replication is a topic which is discussed in several chapters (6,9,10). It seems accepted that tunneling is one avenue to DNA mutation; possibly QM has a more meaningful role to play. The capabilities of artificial quantum systems are explored in chapters 11-14. To the extent QM systems are good at mimicking features of living things, this offers more circumstantial evidence.

One of my favorite parts of the book is the inclusion (as chapters 15 and 16) of the transcripts of two staged debates which took place at conferences: one is on the future of quantum computing (from 2003), and one specifically on the topic of whether life utilizes non-trivial quantum effects (2004). Both debates featured good insights and a good deal of wit. At one point in the latter debate, one of the participants, Howard Wiseman, offered his definition of a non-trivial quantum effect as “something that will make a biologist want to go out and, you know, take a second year quantum mechanics course and learn about Hilbert spaces and operators, so that they understand what’s going on. (p.358)” Again, I thought about how something like the Engel, et.al. result would change the debate if it were held again today. Wiseman and Jens Eisert --another debate participant -- contributed a thoughtful paper (Ch. 17) explaining in a more organized format why they were on the skeptics’ side of the discussion.

Stuart Hameroff gets the last word in the book, offering his positive proposals for how it all might work (Ch.18). He sees in the structure of cells, both cytoskeleton and protoplasm, features which could lead to a larger scale participatory quantum biology. As a layperson, I’m not a good one to offer judgment; my feeling is that Hameroff presents a string of plausible ideas which nonetheless link together to form a very speculative edifice. On the other hand, as far as I know he could be right! Until we have more attention on the topic we will be slow to sort through and find out which quantum biological ideas are fanciful and which are on target.

So, I second the editors of this volume in their hope that its publication will provoke further debate and help motivate experimental research into this fascinating subject.